I think it's one thing to do an acting kiss, which is very, very different from a real kiss, and another to being egged on to kiss someone for what is essentially fan service and a pay day for the paparazzi. I think Sarah handled it well. It's not her (or David's) job to titillate us by kissing. David and Sarah kissing in public is a very different thing from Buffy and Angel kissing on film. One is a job, the other is... crossing a line. If she and David had been comfortable with it, that was one thing... Stargate's Michael Shanks and Ben Browder kissed at 2008 comicon. Actually I think they may have done it more than once (not just that comicon).
(from the pictures that follow, it's clear it was a fun moment)
No one should expect that two actors should pucker up to make paparazzi happy, however. We are not owed a thing.